1. Field of the Invention
The invention is in the field of continuous dyeing of textile webs comprising cellulosic fibers with vat dyes.
2. Description of the Prior Art
The continuous vat dyeing of cellulosic fibers such as cotton and rayon alone or in combination with other fibers is an important industrial process.
Representative of trade processes for the continuous vat dyeing of textile webs is the process of Stott et al as described in U.S. Pat. No. 2,487,197. The process of Stott et al, in brief, comprises first the step of padding a clean textile web with an aqueous dispersion comprising a vat dye in the oxidized form, a process called pigment padding in the trade. Thereafter, usually following drying, the web is padded with a pad liquor comprising a reductant. The most commonly used reductant is sodium hydrosulfite combined with an approximately equal amount of sodium hydroxide. Following this process step, called chemical padding in the trade, the web is passed through an essentially anaerobic steamer where the dye is reduced to the sodium hydroxide-soluble leuco form which becomes fixed on the web. After leaving the steamer the dye is reoxidized to the insoluble form, normally by contacting the web with an aqueous oxidizing agent such as chromic acid-acetic acid or sodium perborate. Soaping, rinsing and drying of the web completes the process.
Although it is not the usual practice, it is also possible, in a similar process, to pad vat dyes onto the web in the leuco form. In this process one employs an aqueous pad liquor comprising the vat dye in mixture with a reductant.
In dyeing webs comprising blends of cellulosic fibers with other fibers, it is usual to add other dyes suitable for the noncellulosic fiber. For example, a disperse dye may be added to dye polyester fibers in cotton-polyester blend. Between the steps of pigment padding and chemical padding the disperse dyes are normally fixed by heating.
The above described dyeing processes produce substantial amounts of waste water effluent. The dyeing and finishing segment of the textile industry, in which the instant invention would be employed, is under increasing pressure to reduce its effluent to the environment.